AJAX and Microwaves
A good take on AJAX interface complexity from Sean McGrath, a man whose output I have respected for some time. He is absolutely right - good design is as much about what you leave out as what you put in.
Interface complexity on the microwave is, frankly, pointless toys. It microwaves. That’s it. Search engines are a bit different in some senses - there are additional features in there that might be useful, or vital, if your requirements suit them. Our website search uses google, and we can only do this because of extra features in google.
A key part of UI design hinges on an important distinction in User Interfaces - ease of use versus easy of learning. Web applications, even more than normal desktop apps, have to cater for a world where 99% of their audience will read no instructions. Many of them don’t even know what your application really does. I do this myself - you assess whether you want to use something not by reading the (often lying) website, you just get clicking.
User Interfaces on the web have to be designed primarily for the clueless (and I mean that in a nice way). Everything non-core should be placed somewhere on the interface where it’s clear to advanced users where to get it, and make everything else really really tolerant of user error. This is difficult to do, and requires quite some finesse.